Notonychopidae (Soria, 1989)

Püschel, Hans P., Shelley, Sarah L., Williamson, Thomas E., Perini, Fernando A., Wible, John R. & Brusatte, Stephen L., 2024, A new dentition-based phylogeny of Litopterna (Mammalia: Placentalia) and ‘ archaic’ South American ungulates, Zoological Journal of the Linnean Society 202 (1), pp. 1-50 : 18

Notonychopidae

( Figs 6B–C, 7B)

Notonychopidae currently includes two accepted genera with an exclusively Palaeogene distribution ( Fig. 2; Supporting information, Table S1 ). Notonychopidae was founded to include Notonychops powelli Soria, 1989 from the Rio Loro Formation [age uncertain, although probably pre-Itaboraian SALMA age ( Gelfo et al. 2020b)], an ungulate with great development of the parastyle in M1–M3, which among other features, makes it anatomically distinctive from any other SANU previously described (Soria 1989b). Soria (1989b) considered that N. powelli was dentally convergent with members of the family Esthonychidae (Tillodonta) , particularly with the lower molars being brachydont and selenodont with a subequal trigonid and talonid length. However, N. powelli shares important anatomical features with the dentition of ‘amilnedwarsids’ and indaleciids, and of the ear region with indaleciids, which madeSoria (1989b) place it within the order Notopterna (see Amilnedwardsidae and the order Notopterna section for more details). Key differences between notonychopids with indaleciids are the absence of enamel fossettes in the P2–M3 and a greater development of the parastyle in the former, among other features (Soria 1989b). McKenna and Bell (1997) later classified Notonychopidae as a family within the order Litopterna , albeit without any anatomical justification ( Table 1 ). Bonaparte and Morales (1997) added to Notonychopidae the taxon Requisia vidmari Bonaparte and Morales 1997 from Punta Peligro (Peligran SALMA) which they found to be phylogenetically close to Wainka tshotshe ( Fig. 1D). However, since this analysis, Goin et al. (2022) called into question an isolated m3 (UNP 946) previously assigned to Requisia vidmari by Bonaparte and Morales (1997) because of its bunodont features and how low and different its cristids are to what would be expected for a notonychopid, suggesting it could be a member of Didolodontidae .

Bonaparte and Morales (1997) failed to find a monophyletic Notonychopidae , as N. powelli and R. vidmari were not as closely related in their phylogeny ( Fig. 1D). In addition, as notonychopids were found nested within Litopterna , most authors now consider them litopterns (e.g., Gelfo et al. 2016, Croft et al. 2020, Goin et al. 2022), which is problematic given that Bonaparte and Morales (1997) used a matrix that includes exclusively litopterns and did not test alternative relationships. Since the early phylogenetic study of Bonaparte and Morales (1997), there has not been any attempt to test the monophyly of Notonychopidae and to resolve its position among litopterns and other SANUs ( Table 2 ).

As the only two known members of Notonychopidae are R. vidmari and the potentially pre-Itaboraian SALMA N. powelli , the temporal interval of this family is not well constrained with a current range of ~63.8–51.4 Mya, and is likely much narrower in range ( Fig. 2B; Supporting information, Table S1 ; Woodburne et al. 2014b, Krause et al. 2017, Gelfo et al. 2020b).